Monazite and rutile occurring in hydrothermally altered W mineralizations, in the Echassières district of the French Massif Central (FMC), were dated by U-Pb isotopic systematics using in-situ Laser ablation-inductively coupled plasma–quadrupole mass spectrometry (LA-ICP-MS). The resulting dates record superimposed evidence for multiple percolation of mineralizing fluids in the same area. Cross-referencing these ages with cross-cutting relationships and published geochronological data reveals a long history of more than 50 Ma of W mineralization in the district. These data, integrated in the context of the Variscan belt evolution and compared to other major W provinces in the world, point to an original geodynamic-metallogenic scenario. The formation, probably during the Devonian, of a quartz-vein stockwork (1st generation of wolframite, called wolframite “a”; >360 Ma) of porphyry magmatic arc affinity is analogous to the Sn-W belts of the Andes and the Nanling range in China. This stockwork was affected by Barrovian metamorphism, induced by tectonic accretion and crustal thickening, during the middle Carboniferous (360 to 350 Ma). Intrusion of a concealed post-collisional peraluminous Visean granite, at 333 Ma, was closely followed by precipitation of a second generation of wolframite (termed “b”), from greisen fluids in the stockwork and host schist. This W-fertile magmatic episode has been widely recorded in the Variscan belt of central Europe, e.g. in the Erzgebirge, but with a time lag of 10–15 Ma. During orogenic collapse, a third magmatic episode was characterized by the intrusion of numerous rare-metal granites (RMG), which crystallized at ~310 Ma in the FMC and in Iberia. One of these, the Beauvoir granite in the Echassières district, led to the formation of the wolframite “c” generation during greisen alteration.
Leaching of uranium-fertile granites represents a major source of uranium, as uraninite is easily dissolved in oxygenated aqueous solutions. This phenomenon is well documented at surface conditions, but remains poorly documented for granites at depth. In this study, we propose that surface-derived oxidized hydrothermal fluids leached uranium from uraninite in the Questembert peraluminous granite at temperatures greater than ~70° to 160°C. This Variscan synkinematic granite is characterized by widespread and pervasive development of vertical and permeable C-S structures. These structures likely facilitated the infiltration of oxidized hydrothermal fluids from the surface, their circulation at depth, and the subsequent fluid-rock interaction in the granite. Published oxygen isotope data shows that it has undergone subsolidus fluid-rock interaction, dated between 312 and 303 Ma by 40Ar/39Ar analyses on muscovite. These interactions were responsible for the concomitant decrease of the feldspar δ18O values together with uranium leaching. Mass-balance calculations suggest that this hydrothermal event could have liberated several hundred thousand tonnes of uranium from the Questembert granite. The liberated uranium may have been dispersed in Early Permian intramountainous basins and therefore disseminated over large areas. This study emphasizes that the efficiency of uranium leaching may be directly related to the extent of subvertical structure development in granites emplaced along strike-slip shear zones, which allow for downward infiltration of oxidized surface-derived fluids. A specific and systematic sampling is required to better constrain the proposed model.
The reassessment of the rare earth element (REE) potential of France led us to investigate REE behaviour in black-shales belonging to the Middle Ordovician Angers-Traveusot formation from central Britany (France), with an emphasis on the formation of nodular grey monazite during the diagenetic and low-grade metamorphic evolution. Temperatures conditions and mass transfer underwent in the black shales were first characterized using rock geochemistry, rock-eval pyrolysis and by gradual changes of clay mineral crystallinity. Then, monazite texture, composition and U–Pb in-situ dating were determined and correlated with the diagenetic/anchimetamorphic conditions. In the Ordovician black shales, nodular monazite appears at the transition between the upper diagenesis and the anchizone metamorphic facies, at conditions between 140 and 250°C, in response to processes controlled by different proxies of competing influences such as the organic matter maturation, Fe oxide/hydroxide and clay transformation with fluid releasing. Monazite occurs mainly as elongated nodules, up to 2 mm in diameter that are mostly characterised by their grey colour due to abundance in host-rock mineral inclusions. Monazite nodules compositions are systematically low in Th and U contents but are zoned with Nd and middle REE rich cores surrounded by light REE-rich rims, with no evidence of inherited domains. More rarely, small grains of LREE-rich monazite are observed in late stage fractures. Monazite nodules were dated at ca. 405-400 Ma, which is proposed to record high heat flux responsible of the anchimetamorphic conditions recorded at the base of Angers-Traveusot formation. Late monazite-Ce were dated at ca. 385 and 350 Ma at the onset of the Variscan deformation.
Reassessment of France's rare earth element (REE) potential led us to investigate REE behaviour in the black shales of the Middle Ordovician Angers-Traveusot Formation in central Brittany (France). This study focuses on the distribution of nodular grey monazite (up to 200 g/t) within the shales, which formed in response to the diagenetic and low-grade metamorphic evolution of the studied area. Whole rock geochemistry, rock–eval pyrolysis and evolution of clay mineral crystallinity were firstly used to determine temperature and mass transfer conditions in the black shales. Then, monazite texture, composition and U–Pb in-situ dates were determined, and correlated with the diagenetic/anchimetamorphic conditions. Nodular monazite appears in the Ordovician black shales at the transition between high-grade diagenesis and the anchizone metamorphic facies, in response to processes controlled by competing influences such as organic matter maturation, Fe oxide/hydroxide reduction and clay transformation with accompanying fluid release. Monazite occurs mainly as elongated nodules, up to 2 mm in diameter that are characterised by their grey colour caused by an abundance of host-rock mineral inclusions. Monazite nodule compositions are zoned with Nd and middle REE-rich cores surrounded by Ce-rich rims, with no evidence of inherited domains. Ce-monazite also occurs as a replacement of nodular monazite or in late-stage fractures. Zoned nodular grey monazites were dated at ca. 403.6 ± 2.9 Ma, which is proposed to record the high heat flux that led to the anchimetamorphic conditions found at the base of the Angers-Traveusot formation, prior to the Variscan deformation. Crystallisation of the metamorphic Ce-monazite occurred at two periods, dated at 382.6 ± 2.9 Ma and 349.6 ± 6 Ma, which correspond to pre-collisional and collisional tectono-metamorphic stages respectively. Nodular grey monazite constitutes an interesting alternative economic solution because of its very low content in both Th (X¯=2160 ppm) and U (X¯=145 ppm) and negligible radiological impact if mined. However, placers currently display a limited economical interest.
High-phosphorus peraluminous rare-elements granites and rare-elements LCT (Lithium, Caesium, Tantalum) pegmatites are the most important sources of raw materials for some critical metals like tantalum (1,2)represent important economic storehouses for industrial minerals like feldspar, quartz, mica or kaolin. They principally emplace in orogenic settings (3). Afast overview of three mainEuropean Variscan districts, i.e. the Moldanubian domain of the Bohemian massif, the French Massif Central (FMC) and the NW Iberia provides a basis for questioning the origin of rare-elements magmatism and the actual classification of rare-elements pegmatites, in particular the LCT pegmatites. Granitic pegmatites are widespread in most of the Bohemian Massifbut LCT pegmatites are most common in the Moldanubian domain. In this area, their emplacements seem mainly controlled by migmatitic domes and shear zones and correspond to two events(4). The older at ~ 333 ± 3 Ma just follow HT-MP event of the end of the Moravo-Moldanubian phase and the younger at ~ 325 ± 4 Ma is contemporaneous with beginning of the Bavarian phase (U-Pb ages on colombite and tantalite). In the FMC, most of the actually known rare-elements magmatic bodies form a province in the North Limousin area, which represents the northwestern part of the FMC.U-Pb dating of columbite-group minerals from Beauvoir, Montebras and Chedeville rare-elements magmatic bodies leads to emplacement ages at 317 ± 6 Ma, 314 ± 4 Ma and 309 ± 5 Ma respectively. The contemporaneous Marche fault system (5), which crosscuts in a general E-W trend all the northern part of the Limousin, seems to be a key-structure for the rare-elements magmatism of the area.
The study area is located in the Galicia-Tras-os-Montes Zone (GTMZ zone, Arenas et al. 1986; Farias et al. 1987 Fig. 1) a part of the Iberian hercynian massif. The GTMZ belongs to the internal zone of the Hercynian belt and is composed of a relative autochthonous and parautochthonous units overthrusted by allochthonous complexes. Studied area is located in the Schistose Domain (parautochthonous Marquinez Garcia 1984) which is composed by a monotonous sequence of schists. Rocks of this domain exhibit a well-developed regional schistosity related to nappes emplacement (D1 and D2 events) and are affected by NS-trending crenulation and folds (D3 event) characterized by a high-temperature metamorphism leading to local development of migmatites. Four generations of granites (G1 to G4), well identified in NW Spain by their textural, geochemical characteristics and crosscutting relationships are present in the studied area. G1 to G3 granites are coeval with late with D3 event. G1 granites are syn-kinematic porphyric biotite granites. G2 granites are syn-D3 two micas granites and leucogranites (Capdevila and Floor 1970; Barrera Morate et al. 1989). G3 granites are biotite-dominant two mica granites (Barrera Morate et al. 1989). G4 granites are post D3 (Capdevila and Floor 1970; Bellido Mulas et al. 1987; Barrera Morate et al. 1989). Gold mineralizations are spatially associated with G3 granites, and Brues, the main deposit, is located on the North-western edge of the Boboras granite roof (fig. 1). Sn-W deposits are represented, by disseminated and vein-type mineralizations. Sn,Ta,Li,Nb±W disseminated mineralizations are hosted by REE-pegmatites-aplites (Fuertes-Fuente and Martin-Izard 1998), crosscut by Sn-bearing quartz veins, spatially associated with G2 granites. The main pegmatite field is the Couso district, located on the Eastern edge of the La Estrada-Cerdedo G2 granite (fig 1). Sn-W±Ta±Nb vein-type deposits are also spatially associated with G2 granites. The most important deposits are located on the Eastern edge of the composite G1-G2 Beariz granite (fig 1).
The study area is located in the Galicia-Tras-os-Montes Zone (GTMZ zone, Arenas et al. 1986, Fig. 1), that belongs to the internal zone of the Hercynian belt and is composed of a relative autochthonous and parautochthonous units overthrusted by allochthonous complexes (Ribeiro et al. 1990). This domain of Palaeozoic schists is affected by a low to high temperature – medium pressure metamorphism. These rocks exhibit a well-developed regional schistosity related to nappes emplacement (D1 and D2 events) and are affected by NS-trending crenulation lineation and folds (D3 event) The late D3 event is characterized by a high-temperature metamorphism leading to development of local migmatite. Both parautochthonous and allochthonous units are intruded by syn- and post-kinematic plutons. Four generations of granites (G1 to G4) are identified by their textural and geochemical characteristics and by crosscutting relationships. Gold deposits are spatially associated with the G3 granites whereas Sn-W deposits are represented, in the study area, by disseminated and vein-type mineralization spatially close to the G2 granites (fig. 1).
Amongst the silicate-rich crystalline rocks that are produced in the continental crust, pegmatites are characterised by their large crystals which give them both an aesthetic and economic interest. Pegmatites crystallise either from fractionated magma derived from a parent granitic body or from the partial melting of meta-sediments or meta-igneous rocks (e.g. amphibolite). The mechanism of residual magma (or fluid) extraction from the parent granitic body has been thoroughly studied, but pegmatitic melt extraction after partial melting has received less attention. We present here a series of non-dimensional numerical experiments using a two-phase flow formulation that couples the Stokes problem to/with non-linear Darcy flow. This approach makes it possible to predict the movement of fluid inclusions (named porosity) in a deformable of a viscous rocks (named porous matrix). We find that the simulation produces either clusters or an isolated body of fluid inclusion depending on the compaction/decompaction ratio of the effectively viscous matrix in which they rise. Using a review of pegmatite natural properties, we propose a scaling of our numerical simulations that describes the ascent of a pegmatite-forming melt produced by partial melting. We then discuss possible travel distances and temperature effects. To discuss our results in light of field observations, we assume that the compaction-decompaction ratio is an accurate proxy for the influence of brittle processes at a scale smaller than the representative volume element, and therefore corresponds structural level variations at which pegmatites are emplaced. We find that our numerical simulation explain the statistical organisation, in terms of level of emplacement, of real fields of pegmatites possibly derived from partial melting of meta-sediments. Pegmatites in fact tend to organise as clusters around brittle faults in upper crustal levels, whereas they present a scattered distribution at mid to lower crustal levels. Our results therefore show that porosity waves are a possible mechanism for rapidly extracting and transporting pegmatite melts formed during low-degree (ca. 10%) partial melting at distances up to a few kilometres in the crust.
